1) we have a couple of office branches (say 20) located in different
parts of the country, employees travel often between these branches

2) each branch office is connected to internet using rather slow /
asymmetrical ADSL

3) the task is to add / delete users on one central machine

4) then, when user is added / deleted, all the changes are "uploaded" to
each branch office - so that authentication there (in branch office)
could be done fast (no need to check on the central server), and
wouldn't be prone to network failures (when link to the central server
is broken)

5) moreover, in each branch, clients should be able to change their
passwords, and Samba should be able to add machines to the database
automatically. These changes should be added to the local machine (to
save time on a tight bandwidth, prevent failures due to broken link
etc.), which would then be transferred to the central server, and from
there, to the rest of the branch offices.